As an air mattress for, for example, bedridden patients, an air mattress having a two-layer structure of an upper-layer adjustable cell section and a lower-layer fixed cell section is described in, for example, PATENT DOCUMENT 1. In this air mattress, each of the adjustable cell section and the fixed cell section is configured by arranging a large number of elongated air cells in parallel. In the fixed cell section, all the air cells communicate with each other, whereas in the adjustable cell section, the air cells are divided into groups which communicate with each other. In use, the fixed cell section is always in an expanded state by supplying air to all the air cells, and the adjustable cell section repeats expansion and contraction in units of groups by alternately supplying and discharging air to/from the air cells in units of the groups. Consequently, the expansion and contraction of the adjustable cell section disperses the body pressure of a patient lying on the air mattress, thereby preventing decubitus ulcers. In addition, the expanded fixed cell section supports the adjustable cell section, resulting in that even when the patient sits up on the air mattress, a bottom hitting phenomenon in which the buttocks of the patient reach the bottom surface of the bed can be prevented.    PATENT DOCUMENT 1: Japanese Patent Publication No. H08-164169